How Do I Rig This Model Using Rigify?
This character is quite complex, and requires a lot of work to get rigged, because it's maden of lots of spare parts and intersecting geometry. Some geometry requires to be organically deformed, while some others parts are supposed to be rigid, so it's impossible to obtain a good result with automatic weights, even if you get it to work without bone heat failings.
My suggestion is to enter edit mode, use the L and Ctrl L functions to select all blocks of vertices that are part of a piece and separate them (pressing P). Then some of theese new spare parts can be weighted automatically, some others will need to be manually weighted. You will find all Rigify Deforming bones in the 30th bone layer.
After the procedure is done you can rejoin them (Ctrl J), the weights will be carried intact.
In the picture an example on the Left leg, which still needs some more refinements.